Concave grating spectrometers separate light by wavelength so as to permit the measurement or recording of one or more of the wavelengths. In their simplest form they consist of an optical detector; a concave grating; and an entrance slit through which a beam of light is directed to the grating. The grating consist of a spherical reflective surface covered with a fine pattern of grooves, generally parallel and evenly spaced. The curved grating surface acts both to separate the wavelengths and to focus the light.
The focusing property of a concave grating is both good and bad. It is good in that it does not require auxiliary focusing devices, such as spherical mirrors, to focus the light on the detector of the spectrometer. This leads to low cost, instrument simplicity and, in the vacuum ultra-violet (UVU) wavelength region, avoidance of light loss mechanisms associated with the auxiliary devices. However, the bad part of a concave grating is that the design is so simple there are very few parameters that can be adjusted to compensate for various optical difficulties.
With the original type of concave gratings, which are made mechanically by scribing or ruling the grooves in a soft metal surface, a common limitation is astigmatism. This is the condition where the focal curve where the images of various wavelengths are narrowest, called the horizontal focal plane, is significantly different from the curve where the same images having minimum height, called the vertical focal plane. Astigmatism causes light losses, and increases the severity of several other aberrations. This problem is worse as the two focal curves are positioned further away from each other.
An improvement over mechanically ruled concave gratings is holographic gratings. Here, the grooves are made by means of an interference pattern formed with a coherent laser. A laser beam is split into two portions which are brought to a focus at two points. The light diverging from the two focal points is allowed to illuminate a spherical surface, coated with a photosensitive surface. Following exposure, the surface is chemically etched, and then coated with a reflective material. This process, having four parameters defining the locations of the two laser focal points, is more general than a mechanically ruled surface, which is only characterized by one parameter, the groove spacing.
A benefit of holographic gratings is that, by adjusting the locations of the laser focal points, the shape and size of the two focal curves can be modified. It was recognized very early that the horizontal focal curve can be made flatter, to better correspond to the nearly flat shape of the vertical focal curve, and that, to a certain extent, the two curves could be made to approach and even cross more than once. An advantage of these crossover points is that the astigmatism is zero for images formed here.
In the article "Aberration-Corrected Concave Gratings Made Holographically" by Cordelle et. al., published in the book OPTICAL INSTRUMENTS AND TECHNIQUES, 1969 (Oriel Press, London), pages 117-124, grating arrangements are described which have three crossings of the two focal curves. However, these designs suffer from some serious limitations. One of the three crossing points is used for the entrance slit, and so is unavailable as a image location. Also, there are only three possible locations for the laser focal points, greatly restricting the possible designs.
At least since the publication "Ray Tracing Through Holographic Gratings", by H. Noda et al, JOURNAL OF THE OPTICAL SOCIETY OF AMERICA, vol. 64, 1974, pages 1037-1042, which developed an exact ray-tracing procedure for concave holographic grating spectrometers, it has been known that, in principle, a computer program could be used to search all possibilities for the best optical design. This approach is limited by several well-known difficulties, such as premature termination of the search at a point representing a design which is locally optimal, but not the best overall design. An example of this approach is shown in a paper by Wayne R. McKinney et al, APPLIED OPTICS, vol. 26, August 1987, page 3108. It was their purpose to design a spectrometer of moderate resolution, with a flat focal plane of specified length, for a given wavelength range. They attempted to find the best design by means of computer-based optimization of the various optical parameters. However, this did not provide the most superior design for their purpose.
As shown in U.S. Pat. No. 4,279,511, there are cases with a concave grate spectrometer where it is advantageous to move a detector along a straight line, so that different wavelengths can be intercepted by the detector. In general, a concave grating, even a holographic grating, is not well suited for this, since the horizontal focal curve is not straight, leading to defocus errors. Additionally, the vertical focal curve, while nearly straight, is not in general disposed along the horizontal focal curve, leading to astigmatism errors. Therefore, it would be desirable to provide a concave grating spectrometer with a flat focal field, wide angular range, low astigmatism, high resolution and high optical throughput.